1. Field of the Invention
The present invention relates to a heater sealed with a carbon wire heating element therein and more particularly to a heater sealed with a carbon wire heating element in which a carbon wire of carbon fibers having a particular crystal structure is sealed as a heating element, resulting in obtaining a heater sealed with a carbon wire heating element excellent in stable resistance when the current is flowing and particularly in anti-aging characteristic.
2. Description of the Related Art
Heaters using a carbon heating element therein have excellent properties in raising and lowering the temperature due to a small heat capacity as compared to heaters using a metal heating element, and offering a wide industrial application to various types of heating apparatus.
In the field of the semiconductor manufacturing, especially, silicon wafers are subjected to various heat treatments in the process thereof. Moreover, such heat treatments are in need of strict temperature control while it is essential to maintain a clean atmosphere for heat treatment in which no particles such as dust or the like are present. For this purpose, it is required to meet various conditions; that is, the heater used for the heat treatment must show an excellent uniformity in heating performance and the heater will not give off contaminating substances during the process.
In general, heaters having a structure in which carbon heating elements are sealed within support members of quartz glass material together with the atmosphere of a non-oxidizing gas are widely used as a heater for semiconductor manufacturing processes.
The inventors developed, in the past, a heater for semiconductor heat treatment apparatus and proposed the same in Japanese Patent Application H10-254513, in which fine carbon fibers are bound into a carbon bundle and a plurality of such bundles are woven into a carbon wire heating element to be filled in the quartz glass support member together with a non-oxidizing gas.
This type of carbon wire heating element not only shares the advantages required for the carbon heating element such as an excellent property to raise and lower the temperature and a high temperature resistance but also an extra advantage of having more flexibility than the heating elements made of solid carbon material because said carbon wire heating element is manufactured by binding a plurality of fine carbon single fibers into a bundle and weaving a plurality of such bundles to such an extent that various structures and configurations are easily realized.
Therefore, the heater having a clean and heat-resistant support member of high purity quartz glass support member in which this type of heating element is sealed together with a non-oxidizing gas is free from a risk of giving off particles and is most suitable to the semiconductor manufacturing heater.
It is to be noted here that the carbon wires used for the conventional heater of this type were manufactured by following a baking process to carbonize the fiber form precursor at a temperature in the neighborhood of about 1300xc2x0 C. and that the quality of the product is maintained by improving the purity of materials to suppress the concentration of contained impurities thereof at a level of 10 ppm or less.
Recently, however, it was recently observed that said conventional carbon wires for the heating element were often used at a temperature higher than the prescribed temperature needed for firing and the disadvantage that the heater performance such as electric resistance thereof or the like becomes unstable has been pointed out.
More specifically,
{circle around (1)} The crystal structure of carbon single fibers composing the carbon wire gradually changes under the heat to lower the carbon wire resistance (xcexa9/m) gradually to such an extent that a change in the resistance of the whole heater due to aging takes place; and
{circle around (2)} The chemicals adhered to the carbon wires are liberated by the heat in the form of a gas to change the inner pressure inside the quartz glass member filled with the carbon wires during the long service life thereof with the result that the performance of the heating element is affected.
It is not rare that such heater sealed with the carbon wire heating element as being put to the industrial use including said heater for the heat treatment of semiconductor wafers has an instantaneous temperature as high as 1800xc2x0 C. when the current flows. Therefore, any solution of such problem has been much in demand.
The present invention is made to solve the problems present in the conventional carbon wire heating element and its object is to provide an excellent heater sealed with a carbon wire heating element which has a stable resistance when the current is flowing and particularly a stable resistance free from aging; that is, the heating performance of the heater will not change throughout a long service period.
Therefore, the present invention provides a heater sealed with carbon wire heating element comprising a heating element composed of carbon wires each being prepared by knitting carbon single fibers into a knitted cord or a braid, each carbon single fiber having a crystal structure with a interlayer spacing d (002) thereof being 0.343 or less and a crystallite size Lc (002) thereof being 4.0 nm or more; and a quartz glass member sealed with said heating element.
Further, the present invention also provide a heater sealed with carbon wire heating element, in a preferred mode of embodiment, characterized in that said heating element is prepared by binding a plurality of carbon single fibers into a bundle, each single fiber having a diameter of 5 through 15 xcexcm and knitting a plurality of such bundles into a knitted cord or a braid.
Specifically, a heater sealed with carbon wire sealing element which is characterized in being prepared by binding 3000 through 6000 carbon single fibers into a bundle and knitting 6 through 12 such bundles into a knitted cord or a braid having a diameter of 1.5 through 3.0 mm is proposed.
Still further, the present invention provides a heater sealed with carbon wire sealing element which is characterized in that said carbon single fibers contain ash contents of 1 ppm or less.
Still further, the present invention provides a heater sealed with carbon wire sealing element which is characterized in that said heating element composed of carbon wire prepared by knitting carbon single fibers into a knitted cord or braid is subjected to a heat treatment at a temperature of 2000xc2x0 C. or more in an inert gas atmosphere.
Still further, the present invention provides a heater sealed with carbon wire sealing element which is characterized in that said quartz glass member is provided with two terminals between which said carbon wire heating element is extended, at least one of said two terminals being provided with a mechanism to adjust a tension exerted on said carbon wire heating element.
The structural feature of the heater sealed with carbon wire sealing element according to the present invention is in that the heating element is composed of carbon wires each being prepared by knitting carbon single fibers into a knitted cord or a braid, each carbon single fiber having a crystal structure with a interlayer spacing d (002) thereof being 0.343 or less and a crystallite size Lc (002) thereof being 4.0 nm; and a quartz glass member sealed with said heating element.
Furthermore, said interlayer spacing d (002) and said crystallite size Lc (002) are measured using Gakushin-method for X-ray diffraction of carbon (Japan Society for the Promotion of Science, Committee 117, Carbon, 1963[36], 25).
The carbon wire heating element composed of carbon fibers having a special crystal structure is obtained by subjecting a carbon wire for the ordinary heating element to a heat treatment at a temperature of 2000xc2x0 C. or more in an inert gas atmosphere.
The carbon wire heating element according to the present invention will not undergo any change in the crystal structure thereof in the temperature range (usually 2000xc2x0 C.) at the time of the heater being in use; that is, no change in physical properties such as the electric resistance (Q/m) will take place because the graphitization will not proceed any longer.
Moreover, the growth of the crystal structure is uniform to make the quality of the product homogenized because the general carbon wire is heated uniformly and simultaneously.
Furthermore, most of the gaseous substances adhered to the carbon wire before the heat treatment are detached and removed.
Therefore, the heater sealed with said carbon wire heating element is found to have a high quality and an excellent anti-aging stability of electric resistance or the like while the current is flowing.
Although there is no perfect interpretation concerning why the carbon wire heating element is excellent in the anti-aging stability of electric resistance or the like, the following interpretation is presumed.
To be more specific, the carbonaceous substances having an intermediate structure between the amorphous carbon and the complete graphite crystal structure are in general called carbon materials, under the category of which petroleum cokes, carbon black and other carbon fibers fall.
It is known that said substances include graphitization-easy carbonaceous substances that easily proceed to grow until the graphite crystal structure by heat treatment and graphitization-resistant carbonaceous substances that will not be graphitized easily beyond a certain level where the graphitization stops. Carbon substances are known to belong said graphitization-resistant carbonaceous substances in view of the crystal structure thereof (interlayer spacing, accumulation thickness of the crystal faces) regardless of the starting materials (PAN, PVA, Pitch) thereof.
With general graphitization-easy carbonaceous substances (such as carbon black) other than the above carbon fibers, the graphitization process proceeds under the heat treatment through three graphitization stages including the first stage, the second stage and the third stage up to the final stage (usually at a temperature of 2400 through 3000xc2x0 C.).
More specifically, the graphitization process enters the first stage with the interlayer spacing d (002) being 0.343 nm or less and with the crystallite size Lc (002) being 2.0 through 4.0 nm. Next, it enters the second stage where the interlayer spacing d (002) undergoes little change whereas the crystallite size Lc alone grows. Finally, the process reaches the third stage where the crystallite size Lc undergoes no change whereas the interlayer spacing rapidly decreases (to a degree of 0.336 nm).
However, while belonging to the graphitization-resistant carbonaceous material, the carbon wire composed of carbon fibers has a crystallite size Lc of at most 5.0 through 10.0 nm, unlike the process mentioned above, even if subjected to heat treatment at a temperature as high as 3000xc2x0 C., said crystallite size being smaller than other type of graphitization-resistant carbonaceous material. In addition, the interlayer spacing d (002) is at most 0.339 nm. The graphitization thereof proceeds to develop the crystal texture from the first stage to the third stage, with the crystallite size Lc being stabilized without passing through the second stage where the crystallite size grows.
It is assumed that the carbon fibers subjected to the high temperature treatment shows such characteristic graphitization process because the micro texture of the carbon fibers is composed of a carbon chain structure in which a micro-crystallite portion and a non-crystallite portion are connected to each other in a continuous manner, the majority of said micro crystallite portion are arranged in a uniaxial direction to form a micro texture characteristic in the fibers intertwined each other.
Further, oxygen and other foreign substances contained in the carbon wire are gasified in the first stage of the heat treatment be discharged from the carbon wire.
Therefore, in the carbon wire sufficiently heat treated at a temperature of 2000xc2x0 C. and preferably 2020xc2x0 C. or more, the crystal structure of the carbon fiber composing the same proves to have the above specific structure. On the other hand, the graphitization will not fully proceed at a temperature less than 2000xc2x0 C. with the result that it is difficult to properly adjust various physical properties including the resistance thereof (xcexa9/m).
Further, the carbon fibers subjected to said graphitization have larger elongation under tension than prior to said treatment.
The carbon wire prepared by knitting carbon fibers into a knitted cord or a braid have carbon single fibers contacting each other at their peripheries such that the carbon wire becomes to have less electric resistance when tension is exerted thereon in the longitudinal direction thereof and less resistance fluctuation taken in the longitudinal direction because of increased contact area.
Therefore, the carbon wire of the present invention is preferably sealed in the quartz glass member in a taut condition (under tension).